From ab12a66c7262bb1c2b0bc572bc21398c76e07cdd Mon Sep 17 00:00:00 2001
From: Sam Harwell <sam@tunnelvisionlabs.com>
Date: Tue, 20 Oct 2020 07:54:20 -0700
Subject: [PATCH] Optimize FixedArray8<T> indexer

* Use `Unsafe.Add` for direct access
* Apply MethodImplOptions.AggressiveInlining
* Use throw helper
---
 .../Immutable/FixedArray8`1.cs                | 78 ++++---------------
 .../ThrowHelper.cs                            | 17 ++++
 .../TunnelVisionLabs.Collections.Trees.csproj |  1 +
 3 files changed, 32 insertions(+), 64 deletions(-)
 create mode 100644 TunnelVisionLabs.Collections.Trees/ThrowHelper.cs

diff --git a/TunnelVisionLabs.Collections.Trees/Immutable/FixedArray8`1.cs b/TunnelVisionLabs.Collections.Trees/Immutable/FixedArray8`1.cs
index 4c7ccd7..ae27c96 100644
--- a/TunnelVisionLabs.Collections.Trees/Immutable/FixedArray8`1.cs
+++ b/TunnelVisionLabs.Collections.Trees/Immutable/FixedArray8`1.cs
@@ -7,10 +7,14 @@ namespace TunnelVisionLabs.Collections.Trees.Immutable
     using System.Collections.Generic;
     using System.Diagnostics;
     using System.Runtime.CompilerServices;
+    using System.Runtime.InteropServices;
 
+    [StructLayout(LayoutKind.Sequential)]
     internal struct FixedArray8<T>
     {
         private T _item0;
+#pragma warning disable IDE0044 // Add readonly modifier
+#pragma warning disable CS0169 // The field 'name' is never used
         private T _item1;
         private T _item2;
         private T _item3;
@@ -18,83 +22,29 @@ internal struct FixedArray8<T>
         private T _item5;
         private T _item6;
         private T _item7;
+#pragma warning restore CS0169 // The field 'name' is never used
+#pragma warning restore IDE0044 // Add readonly modifier
 
         public int Length => 8;
 
         public T this[int index]
         {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get
             {
-                switch (index)
-                {
-                case 0:
-                    return _item0;
-
-                case 1:
-                    return _item1;
-
-                case 2:
-                    return _item2;
-
-                case 3:
-                    return _item3;
-
-                case 4:
-                    return _item4;
+                if ((uint)index >= 8)
+                    ThrowHelper.ThrowIndexOutOfRangeException();
 
-                case 5:
-                    return _item5;
-
-                case 6:
-                    return _item6;
-
-                case 7:
-                    return _item7;
-
-                default:
-                    throw new IndexOutOfRangeException();
-                }
+                return Unsafe.Add(ref _item0, index);
             }
 
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
             set
             {
-                switch (index)
-                {
-                case 0:
-                    _item0 = value;
-                    break;
-
-                case 1:
-                    _item1 = value;
-                    break;
-
-                case 2:
-                    _item2 = value;
-                    break;
-
-                case 3:
-                    _item3 = value;
-                    break;
-
-                case 4:
-                    _item4 = value;
-                    break;
+                if ((uint)index >= 8)
+                    ThrowHelper.ThrowIndexOutOfRangeException();
 
-                case 5:
-                    _item5 = value;
-                    break;
-
-                case 6:
-                    _item6 = value;
-                    break;
-
-                case 7:
-                    _item7 = value;
-                    break;
-
-                default:
-                    throw new IndexOutOfRangeException();
-                }
+                Unsafe.Add(ref _item0, index) = value;
             }
         }
 
diff --git a/TunnelVisionLabs.Collections.Trees/ThrowHelper.cs b/TunnelVisionLabs.Collections.Trees/ThrowHelper.cs
new file mode 100644
index 0000000..3b85532
--- /dev/null
+++ b/TunnelVisionLabs.Collections.Trees/ThrowHelper.cs
@@ -0,0 +1,17 @@
+// Copyright (c) Tunnel Vision Laboratories, LLC. All Rights Reserved.
+// Licensed under the MIT License. See LICENSE in the project root for license information.
+
+namespace TunnelVisionLabs.Collections.Trees
+{
+    using System;
+    using System.Diagnostics.CodeAnalysis;
+
+    internal static class ThrowHelper
+    {
+        [DoesNotReturn]
+        internal static void ThrowIndexOutOfRangeException()
+        {
+            throw new IndexOutOfRangeException();
+        }
+    }
+}
diff --git a/TunnelVisionLabs.Collections.Trees/TunnelVisionLabs.Collections.Trees.csproj b/TunnelVisionLabs.Collections.Trees/TunnelVisionLabs.Collections.Trees.csproj
index 436aafd..5eb4bad 100644
--- a/TunnelVisionLabs.Collections.Trees/TunnelVisionLabs.Collections.Trees.csproj
+++ b/TunnelVisionLabs.Collections.Trees/TunnelVisionLabs.Collections.Trees.csproj
@@ -26,6 +26,7 @@
 
   <ItemGroup>
     <PackageReference Include="System.Collections.Immutable" Version="1.4.0" />
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.0" />
   </ItemGroup>
 
 </Project>
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